Meconium is fetal stool, and when passed in utero, it may indicate or be associated with fetal stress. If the concentration of meconium in the amniotic sac is sufficiently high, the fetus may aspirate the meconium into the respiratory system and cause potentially-fatal complications in the airways and lungs.
To date, most clinical caretakers assess the concentration of meconium in amniotic fluid by visual inspection. In this type of analysis, the concentration of meconium is typically characterized as being "thin" (i.e., a relatively low concentration) or "thick" (i.e., a relatively high concentration). Amniotic fluid having a "thick" meconium concentration typically may have the consistency of pea soup and contain particulate matter.
During a typical labor, the amniotic fluid may not be visually inspected until released into the vaginal pool. As a result, there may be a significant time delay between the release of any meconium within the amniotic sac and its detection by the medical staff. In addition, the visual assessment of meconium concentration is a subjective determination, and depending upon the person rendering the assessment, the interpretation may vary and, in turn, lead to misdiagnoses.
One prior art approach to obtaining a more objective assessment of meconium content is described in Weitzner, J. S. et al., "Objective Assessment Of Meconium Content Of Amniotic Fluid", Obstet Gynecol (1990) 76: 1143-1145. The Weitzner et al. technique involves withdrawing amniotic fluid samples, spinning them within a centrifuge, and analyzing a meconium "crit".
U.S. Pat. No. 5,009,507 to Katz shows a method for evaluating meconium content in amniotic fluid wherein a sample of amniotic fluid is visually compared to a meconium concentration chart while transmitting fluorescent light through both the sample and the chart.
U.S. Pat. No. 5,514,598 to Doody shows a method for detecting prenatal meconium excretion by chromatographically separating proteins in a sample of the fluid, and analyzing the separated proteins for the presence of a meconium antigen.
Although these prior art approaches should provide a more objective assessment of meconium concentration than visual inspection, they do not allow for a real-time, continuous reading of the concentration of meconium in the amniotic sac. In addition, these prior art techniques tend to be relatively cumbersome and time consuming, require sophisticated and expensive equipment, and/or may not provide consistent results.
U.S. Pat. No. 5,361,759 to Genevier et al. ("the '759 patent") is directed generally to a non-invasive system for optically detecting in vivo the presence and concentration of meconium and/or blood in amniotic fluid. The system comprises a single probe having a body 32 for insertion into the uterus. The probe includes an optical cell 31 defining a passageway for permitting the flow of amniotic fluid therethrough, and a fiber optic bundle 33 optically connected to the passageway. The fiber optic bundle 33 includes two branches, one for transmitting light into the optical cell, and the other for collecting reflected light and conveying it to a spectrophotometer comprising a plurality of photodetectors. The light reflected off the amniotic fluid within the passageway is returned from the probe to second, third and fourth photodetectors to detect the reflectance values at the following three wavelengths of interest: (i) 405-420 nm (for meconium), (ii) 540 nm (for blood), and (iii) 700 nm (for a baseline). The concentration of meconium and blood is calculated based on the detected values at each wavelength.
One potential drawback of measuring the light reflected off the amniotic fluid as described in the '759 patent is that the intensity of the reflected light may be relatively weak, and therefore the apparatus may have a relatively low sensitivity, particularly within the clinically relevant range for assessing meconium thickness. The system of the '759 patent also requires multiple wavelengths, and appropriate shielding, particularly if the probe is placed, for example, against the uterine wall. These requirements can lead to additional complexity and expense. Another drawback of the approach described in the '759 patent is that the system employs only a single probe and sensor. The meconium concentration may vary from one location to another within the amniotic sac. Thus, the single probe of the '759 patent may accurately detect the absence of meconium, or a relatively low concentration of meconium at its particular location within the amniotic sac, but may not detect the presence of, or a dangerously high concentration of meconium at another location in the amniotic sac. In addition, the single probe employed in the system of the '759 patent may become clogged with relatively "thick" meconium and/or other particulate matter, and in turn become inoperative or otherwise fail to accurately monitor, or assess the concentration of meconium in the amniotic fluid.
U.S. Pat. No. 5,172,693 to Doody ("the '693 patent") shows another prior art system for in vivo, optical detection of meconium in amniotic fluid. The '693 patent does not teach inserting the probe into the amniotic fluid, but rather teaches placing a probe against the outside surface of the uterine wall (FIGS. 2 and 3). Light at selected wavelengths is transmitted through the uterine wall and into the amniotic fluid for exciting the fluorescent pigments of the meconium. The fluorescence emitted by the meconium pigments is detected, spectrally isolated, amplified, and analyzed to determine the presence of meconium in amniotic fluid. Thus, although the '693 patent shows a system for detecting the presence of meconium, it does not teach or suggest a system for measuring or otherwise obtaining a qualitative and/or quantitative assessment of the concentration of meconium in amniotic fluid.
Accordingly, it is an object of the present invention to overcome one or more of the above-described drawbacks and disadvantages of the prior art.